The material of choice for heat exchangers in LNG and LPG plants is aluminum. Aluminum is chosen primarily because of its high thermal conductivity, ease of fabrication, reasonable cost and availability. A difficulty arising from the use of aluminum to construct heat exchangers for LNG plants is the fact that all natural gas contains measureable amounts of mercury. Mercury is hazardous to aluminum equipment because the mercury promotes rapid deterioration of the aluminum once the mercury penetrates the coating of aluminum oxide usually present on aluminum surfaces. The aluminum becomes embrittled and is subject to failure when tensile stresses are applied.
Attempts to cope with the problem of mercury contamination in natural gas have been based on either replacing the equipment from time to time, flaring the gas, or diluting it with an inert gas. None of these approaches has been particularly successfully either from a technical or economic view point.
Plants processing gases of relatively high mercury concentration, for instance greater than one part per billion, usually treat incoming feed gas to reduce the mercury concentration to less than 0.1 parts per billion. Nevertheless, over a number of years of operation mercury accumulates on the aluminum surfaces in sufficient quantity to become potentially hazardous and engender failure.
Ordinarily the incoming feed gas is passed through beds of activated charcoal containing elemental sulfur deposited thereon at moderately elevated temperatures. The elevated temperature is required to promote the direct reaction of mercury vapor and sulfur to form mercuric sulfide. As noted previously, however, some mercury escapes from this treatment system and in time permeates throughout the rest of the plant, particularly in areas where the equipment is made from aluminum or aluminum alloys. A method to convert the aluminum-mercury amalgam formed and the accumulated liquid mercury is a highly desirable objective.
Thus, a primary object of this invention is to provide a physical-chemical treatment for aluminum heat exchangers and other aluminum equipment that has become contaminated with mercury. Another object of the invention is to restore the ductility of aluminum components, particularly aluminum tubing that has been exposed to mercury and thereby damaged. Still, another object of this invention is to decontaminate the aluminum surface during the time when the equipment is not in use, that is during the normal derime time for gas liquefaction equipment. Another object is to accomplish this decontamination without diminishing either mechanical properties or heat transfer properties of the equipment.